The present invention relates to an electrical circuit for automatically controlling an input current to a current translating device having an output current functionally related to the input current.
Current translating devices, such as current mirrors, current transducers, opto-couplers, and the like, produce an output current that is functionally related to an input current. The functional relationship between the input and output currents of these devices can be defined as a current-transfer ratio wherein a given amount of input current produces a proportional amount of output current. Knowing the current-transfer ratio for a particular type of current translating device permits the design of control circuits that establish the input current level and correspondingly the output current level for the device. However, the current-transfer ratios of similar types of devices can vary over a wide range due to variation in the electrical characteristics of the individual elements making up the device. Additionally, the current-transfer ratio of a given device can vary over time due to changes in temperature, degradation of the device, and so forth. Further, it is desirable in some applications to controllably change the current-transfer ratio of a device to produce a variable current output.
An opto-coupler is an example of a current translating device that exhibits the above described characteristics. An opto-coupler is generally composed of a light source, such as a light-emitting diode (LED), and an opto-receiver, such as a phototransistor. The current input to the LED produces a radiating light output from the LED that is coupled to the phototransistor. A current output is produced by the phototransistor in response to the light intensity from the LED which is controlled by its current input. As previously stated, the individual components of a current-translating device, in this case an optocoupler, have a wide range of electrical characteristics which results in a wide range of current-transfer ratios. It is for this reason that an adjustable control circuit is required for establishing the input current level to the opto-coupler based on its current-transfer ratio. In addition, the current-transfer ratio of the opto-coupler can change if foreign material collects on the LED or the phototransistor and diffuses the light.
A circuit is needed for use with a current translating device having a wide range of current transfer ratios that automatically controls the input current to the device so that the proper, functionally related output current is produced.